1. Field of the Invention
The present invention relates to control of the output voltage of an AC electrical system by means of a converter connected to that system. The present invention is particularly, but not exclusively, concerned with output voltage control in a power control system.
2. Summary of the Prior Art
Consideration has been given to the control of power equipment using a self-commutated converter. Such a converter controls the leading or lagging of reactive power, thereby to control the amplitude and phase of the output voltage of the AC power system. In such an arrangement, it is necessary to provide a control apparatus which monitors current values in the AC power system and provides appropriate control of the converter.
FIG. 1 of the accompanying drawings shows an AC power system to which is connected a known control apparatus having means for monitoring current values in the AC system and an inverter for controlling the amplitude and phase of the output. The resultant structure thus defines an AC electrical system.
In the system shown in FIG. 1, an AC power source 1 is connected via an impedance 2 to a load 3. A capacitor 41 is connected to the DC side of an inverter 42. The inverter 42 is a voltage type self-commutated inverter (converter) comprising switching elements 4201 to 4206 having a self-commutating function. The switching elements 4201 to 4206 may be, for example, gate turn-off thyristors (GTO). Diodes 4207 to 4209 and 4211 to 4213 are connected to the switching elements 4201 to 4206 in anti-parallel. A transformer 43 connects the inverter 42 to the AC system formed by source 1, impedance 2 and load 3. The capacitor 41, inverter 42 and transformer 43 form a reactive power compensator.
The reactive power compensator thus formed is controlled by control units 410 to 423. An AC detection circuit (ACD) 410 detects three-phase alternating currents Iu, Iv, Iw at the connection of the reactive power compensator to the AC system, and a first transforming circuit (TRANS) 411 transforms the three-phase alternating currents Iu, Iv and Iw to two-phase alternating currents Ia, Ib of fixed coordinants, according to Equation 1 below. ##EQU1##
A second transforming circuit (TRANS) 412 transforms the currents Ia, Ib from Equation 1 to direct current signals on axes d and q of co-ordinates according to Equation 2 below, using the voltage phase angle .theta. (=107 t, .omega.=2 .pi.f) of the AC system. ##EQU2##
A first adder (ADD) 413 derives the difference between a current command value Iqp in the q axis and the direct current signal Iq of Equation 2, which is transformed by the second transforming circuit 412. An operational amplifier (AMP) 414 then amplifies the difference derived from the first adder 413. A multiplier (MULT) 415 multiplies the direct current signal Id of Equation 2, which is transformed by the second conversion circuit 412, by the impedance of the transformer 43. A second adder (ADD) 416 subtracts the output of the operation amplifier 414 from the output of the multiplier 415.
A third adder (ADD) 417 derives the difference between a current command value Idp in the d axis and the direct current signal Id, from the second transforming circuit 412. An operational amplifier (AMP) 418 then amplifies that difference. A multiplier (MULT) 419 multiplies the direct current signal Iq, which is transformed by the second transforming circuit 412, by the impedance of the transformer 43. A fourth adder (ADD) 420 subtracts the output of the operational amplifier 418 and the output of the multiplier 419 from a bias signal Vs.
The outputs of adders 416, 420 are fed to a phase angle calculation circuit (PHASE CALC) 412 which derives a command value for the phase angle .delta. of the output voltage of the inverter 42. In a similar way, the outputs of the adders 416, 420 are fed to an amplitude calculation circuit (AMP CALC) 422 for calculating a command signal k for the amplitude of the output voltage of the inverter 42. Those control signals are fed to a pulse width modulation waveform generating circuit (PWM) which generates control pulses to the switching elements 4201 to 4206 of the inverter 42 so as to generate an output from the reactive power compensator which gives appropriate control of the phase and amplitude of the voltage of the AC system.
The operation of the PWM 423 will now be discussed in more detail with reference to FIGS. 2(a) and 2(b). FIG. 2(a) shows a carrier waveform C and a modulated waveform M, and FIG. 2(b) shows switching element control pulses. Those pulses have a value of "1" when the modulated waveform M is greater than the carrier waveform C, and have a value of "0" when the modulated waveform M is less than the carrier waveform C. When the control pulse has a value of "1" the switching element 4201 (or element 4203, or element 4205) is turned on and the switching element 4202 (or element 4204, or element 4206) is turned off. When the control pulse is "0", the switching element 4201 (or element 4203, or element 4205) is turned off on the switching element 4202 (or element 4204 or element 4206) is turned on.
When the amplitude of the command signal k increases, the amplitude of the modulated waveform M increases. As a result, the pulse width of the pulses of FIG. 2(b) increases and hence the AC output voltage of the inverter 42 increases. In a similar way, when the amplitude k of the command signal decreases, the output voltage of the inverter 42 decreases. The signal .delta. representing the phase angle corresponds to the phase angle of the output voltage of the inverter 42 relative to the AC system voltage, and also to the phase angle of the carrier waveform C and modulated waveform M. The reactive power of the reactive power compensator can be obtained by controlling the amplitude of the command signal k when the AC voltage is kept at a constant value. By increasing the value of signal k, leading reactive power is controlled, and by decreasing lagging reactive power is controlled.
The effect of the control achieved by the system of FIG. 1 will now be discussed with reference to FIG. 3, which shows a voltage and current vector diagram. FIG. 3 illustrates the case where a lagging current flows through the reactive power compensator. In FIG. 3, the vectors shown by arrows are as follows:
Vi: The output voltage of the inverter 42 PA1 I: Current PA1 X: Impedance PA1 Vs: The voltage of the AC system
In FIG. 3, the direction of the d axis is the same of that of the system voltage Vs, so that the direction of the q axis lags by 90 electrical degree relative to the d axis. The inverter voltage Vi is expressed by Equation 3 below, referring to the d and q axis signals. ##EQU3##
Thus, the phase calculation circuit 421 and the amplitude calculation circuit 422 command values on the basis of Vi and .delta. of Equation 3, to create appropriate control signals to the PWM 423. The control devices 413 to 416 control the command values for the component in the q axis and the control devices 417 to 420 control the command values for components in the d axis. Vs is equivalent to the AC system voltage and the values Iqp and Idp are command values of Iq and Id in FIG. 3.
Thus, the currents are decomposed into components in the d axis and the q axis respectively, so that those components can be controlled independently and it is possible to control the currents to a suitable target value, with a high speed of operation. Such control is discussed in more detail in the article by Y Tokiwa et al. entitled "Application of a digital instantaneous current control for static induction thyristor converters in the utility line" published in PCIM 1988 Proceedings.